Designing Nanostructured 3D Printed Materials by Controlling Macromolecular Architecture

Abstract Nanostructured polymeric materials play important roles in many advanced applications, however, controlling the morphologies of polymeric thermosets remains a challenge. This work uses multi‐arm macroCTAs to mediate polymerization‐induced microphase separation (PIMS) and prepare nanostructured materials via photoinduced 3D printing. The characteristic length scale of microphase‐separated domains is determined by the macroCTA arm length, while nanoscale morphologies are controlled by the macroCTA architecture. Specifically, using 2‐ and 4‐ arm macroCTAs provides materials with different morphologies compared to analogous monofunctional linear macroCTAs at similar compositions. The mechanical properties of these nanostructured thermosets can also be tuned while maintaining the desired morphologies. Using multi‐arm macroCTAs can thus broaden the scope of accessible nanostructures for extended applications, including the fabrication of actuators and potential drug delivery devices.


Resin preparation
Resins were prepared following the values reported in Table S1. The wt% of macroCTA were selected based on our previous study for potential comparison. [1] For instance, 10 g of resin R1-90-28.2 is consists of 2.82 g monofunctional linear macroCTA (Xn = 90), 3.82 g AA, 3.31 g PEGDA 250 and 0.05 g TPO. These components were vortex mixed for 30 s, followed by a 10 min bath sonication (Hwashin powersonic 410, 400 W). The obtained homogeneous mixture was then directly used for 3D printing. For non-PIMS resin counterparts, macroCTA was replaced by equivalent amount RAFT agent corresponding to 1-arm, 2-arm and 4 arm for R1, R2, R4 based resins respectively) and BA monomer.

3D printing procedures
Unless specifically mentioned, all samples for AFM, SAXS, DMA, tensile testing, and the swelling study were 3D printed using a DLP 3D printer (Anycubic Photon S) with a violet light LED array (λmax = 405 nm, I0 = 0.4 mWcm -2 ). Samples were 3D printed using a layer thickness of 100 μm and curing time per layer of 180 s. The printed objects were separated from the build stage, washed with ethanol, air dried, and post-cured under violet light (λmax = 405 nm, 10 mWcm -2 ) for 15 min before any measurement.
The flower-shaped solvent responsive actuator (0.6 mm in thickness) was 3D printed using a layer thickness Swelling study and swelling-induced actuation of 3D printed flower 3D printed rectangular prisms (L × W × T = 8 × 8 × 2 mm) were examined to determine the swelling properties in water and toluene of materials fabricated using various resins. The rectangular prisms were weighed before swelling to provide W0. Then, they were immersed in 5 mL of water or toluene in 20 mL glass vials for swelling. At fixed time intervals, the samples were weighed after removing the excess solvent by wiping with paper towel to get Wt. The wt% swelling ratios were then calculated by the following Equation 1:

S6
The printed and post-cured multi-material flower was put into a beaker filled with toluene or water, with the PIMS-part of the material facing down. The flower was taken out of the beaker at 10 min increments, pictures were taken, and the flower was returned to the beaker.

Model dye release experiments with 3D printed materials
The studied resins were loaded with 0.1 wt% of 9,10-diphenylanthracene prior to 3D printing. Rectangular Then, a stir bar was introduced to the cuvette to facilitate fluid movement and reduce concentration gradients in the cuvette. UV-Vis spectra of 9,10-diphenylanthracene in toluene released by 3D printed dye-loaded samples were obtained at several time points to allow calculation of the dye release profiles. The concentration of dye in toluene was calculated from a calibration curve of 9,10-diphenylanthracene based on its absorbance at 375 nm.

Characterizations Nuclear magnetic resonance (NMR)
All NMR spectra were recorded on Bruker Avance III 400 MHz spectrometer using an external lock (CDCl3).
Monomer conversion was calculated by comparing integrals of residual monomers (at ~ 6 ppm) and polymers (at 4.05 ppm, a in Figure S2) in NMR spectra ( Figure S2).  Figure S2.

Attenuated total reflectance -Fourier transform infrared (ATR-FTIR) spectroscopy for monitoring polymerization kinetics
ATR-FTIR spectroscopy was performed to monitor photopolymerization kinetics using a Bruker Alpha FTIR spectrometer equipped with room temperature DTGS detectors. After taking a background reading of the empty plate, 20 μL of polymerization resin was pipetted onto the ATR crystal plate. An absorption spectrum was then obtained by scanning the droplet from 400-4000 cm -1 . After an initial reading, the IR absorption spectra were obtained at various times of irradiation with a Thorlabs mounted LED with a collimation adapter (λmax = 405 nm, I0 = 3.7 mW cm -2 ). The vinyl bond conversion was deduced from the disappearance of the C=C bending peak at 1630 cm −1 normalized to the C=O stretching peak at 1760 cm −1 as an internal standard.

S8
As such, the real-time conversion was calculated by Equation 2, where αt is vinyl bond conversion after irradiation of t s, int0 and std0 refer to the integration of peak in the range of 1600-1650 cm -1 and 1670-1800 cm -1 for unreacted sample, intt and stdt refer these two values after irradiation of t s

UV-Vis spectroscopy
All UV-vis spectra were recorded using a Varian Cary 300 spectrophotometer. Scans were conducted in the range of 600-200 nm at 600 nm/min. Resin measurement and dye release experiments were conducted in a 1×1 cm quartz cuvette. A spectrum obtained with an empty cuvette was used for zero/baseline correction.

Atomic force microscopy (AFM)
All AFM measurements were performed on the Bruker Dimension ICON SPM, with a NanoScope V controller (software version 9.70). Mechanical property mapping was performed using peak force tapping mode on the top layer of printed objects using the SCANASYST probe (from Bruker AFM probes). The scan size was set to 1 µm. The scan rate was set at around 0.7 to 0.8 Hz with a peakforce of approximately 500 pN.
The feedback gain was adjusted accordingly to optimize tracking of the specimen surface, without any significant feedback noise. The resolution of the image was set to 512 pixels per line for a 1 µm scan size. For peakforce QNM measurements, the tip was calibrated using the thermal tunning method. AFM images were analyzed using NanoScope Analysis software, version 1.7. For the statistical length analysis, at least 50 areas were carefully traced by hand to determine average domain size and domain spacing using ImageJ software.
Histograms of the size distribution were constructed. Average PBA domain width (D), domain spacing (d) were calculated using Equation 3: Where N is the number of observations and d is the determined size for each measurement.

SAXS fitting using Teubner-Strey (T-S) model
The SAXS data for microphase-separated 3D printed materials were fitted using T-S model [2] in SasView software. According to T-S model Where q = (4π/λ)sinθ, λ is the wavelength, 2θ is the scattering angle; b is background scattering; a2, c1, c2 are fitting parameters used to calculate domain spacing (dTS), correlation length (ξ) and the amphiphilicity factor (Tg) was determined using the temperature at the peak of the tan δ curve.

Tensile testing
Dog-bone specimens were designed using Fusion 360 3D modelling software by modifying the ASTM D638 Type I specimen [3]  The tensile tests were performed using a Mark-10 ESM303 with a 1 kN force gauge. The travel speed for all tests was 1.1 mm/min. All tensile results were performed in triplicate. The tensile stress (MPa) was calculated S11 as the maximum force (N) divided by the initial cross-sectional area (mm 2 ) of the gauge section. The strain was determined as the measured travel distance relative to the original specimen gage length, expressed as a percentage. S12 Additional Data Figure        S5; ecorrelation length determined from T-S fitting using equation S6; f -The ratio of ξ/dTS is a measure of the domain size polydispersity, the smaller the ratio, the larger the polydispersity [4] ; gamphiphilicity factor determined using equation 8.    Additional References: